Engineering Magnetotactic Bacteria as Medical Microrobots

Abstract

Nature's ability to create complex and functionalized organisms has long inspired engineers and scientists to develop increasingly advanced machines. Magnetotactic bacteria (MTB), a group of Gram‐negative prokaryotes that biomineralize iron and thrive in aquatic environments, have garnered significant attention from the bioengineering community. These bacteria possess chains of magnetic nanocrystals known as magnetosomes, which allow them to align with Earth's geomagnetic field and navigate through aquatic environments via magnetotaxis, enabling localization to areas rich in nutrients and optimal oxygen concentration. Their built‐in magnetic components, along with their intrinsic and/or modified biological functions, make them one of the most promising platforms for future medical microrobots. Leveraging an externally applied magnetic field, the motion of MTBs can be precisely controlled, rendering them suitable for use as a new type of biohybrid microrobotics with great promise in medicine for bioimaging, drug delivery, cancer therapy, antimicrobial treatment, and detoxification. This mini‐review provides an up‐to‐date overview of recent advancements in MTB microrobots, delineates the interaction between MTB microrobots and magnetic fields, elucidates propulsion mechanisms and motion control, and reports state‐of‐the‐art strategies for modifying and functionalizing MTB for medical applications.

Magnetotactic bacteria (MTB) are living microorganisms that produce magnetosomes for navigation using the Earth's geomagnetic field. Their built‐in magnetic components, along with their intrinsic and/or modified biological functions, make them one of the most promising platforms for making future living and programmable microrobots. This review highlights recent advances in MTB‐based microrobotics, detailing their interactions with magnetic fields, propulsion mechanisms, motion control, and emerging strategies for engineering and functionalizing MTB for biomedical applications.

1. Introduction

Micro and nanorobots are small‐scale artificial machines capable of performing directed tasks through self‐propulsion or external actuation.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ^{10 ]} Based on the power source, these modes of locomotion are categorized into chemically, magnetically, optically, ultrasonically, electrically, and biologically driven systems.^{[ 11} , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ^{19 ]} Among these systems, magnetic fields are commonly used for remote controllability, fuel‐free biocompatibility, reconfigurability, programmability, recyclability, and multi‐functionality, offering unparalleled advantages.^{[ 20} , ²¹ , ²² , ^{23 ]} Magnetism is also widely used in modern medical devices such as magnetic resonance imaging and magnetic particle imaging, enabling easier integration of magnetically powered micro‐ and nanorobots.^{[ 24} , ^{25 ]} Such versatilities empower magnetic microrobots to undertake diverse tasks, such as targeted drug/gene delivery, cellular manipulation, minimally invasive surgery, targetted gene regulation, disruption/eradication of biological membranes, imaging‐guided treatment, toxin removal, and biosensing.^{[ 15} , ²¹ , ²⁶ , ²⁷ , ^{28 ]} To date, an array of magnetic structures has been designed and synthesized to construct magnetically propelled micro‐ and nanorobots. These magnetic robots come in various forms, including spherical, helical, wire‐like, and flexible structures.^{[ 29} , ³⁰ , ³¹ , ³² , ^{33 ]} However, synthesizing artificial magnetic materials that simultaneously possess robust magnetic moments, excellent dispersibility, satisfactory biocompatibility, and, more importantly, biological functions (intrinsic or modified) remains a technological challenge.^{[ 1} , ^{34 ]}

Magnetotactic bacteria (MTB) contain naturally occurring bio‐magnetic microelements, called magnetosomes, which have evolved under selective pressure.^{[ 35 ]} These magnetosomes exhibit considerable magnetic sensitivity due to their relatively large intrinsic magnetic moments. The ability of MTB to perceive and respond to magnetic fields stems from their endogenous magnetosomes, which are organelles found within the prokaryotic cells.^{[ 36 ]} Magnetosomes are composed of single‐domain Fe₃O₄ or Fe₃S₄ nanocrystals enveloped by biological membranes.^{[ 36 ]} Their chemical composition and crystal properties exhibit high homogeneity due to genetic control and enzymatic catalysis during biogenesis.^{[ 37 ]} Magnetosomes are often arranged into elongated chains, firmly anchored within MTB cells by specific cellular scaffold components.^{[ 38 ]} This arrangement significantly enhances the magnetic induction and responsive behavior of the bacterial cells. These magnetic microelements, as well as flagella rotation, are used to find optimal growth zones within anoxic transition areas in their natural aquatic environments.^{[ 39 ]} Finally, as living cells, MTBs have various proteins, vesicles, and other biological components that can be used and modified for diverse functions in diagnostics and therapeutics. These distinctive advantages, including the presence of magnetosomes, facultative anaerobic characteristics, and robust swimming capabilities driven by bacterial flagella rotation, position MTB cells as competitive candidates for constructing magnetically driven microrobots.^{[ 40 ]}

Recent reviews on MTB have primarily focused on the biomineralization mechanisms of magnetosomes, purification techniques, bacterial cultivation, gene editing, and potential applications such as pollutant adsorption.^{[ 41} , ^{42 ]} Although these reviews provide fundamental insights and the latest developments in MTB research, a comprehensive understanding of how MTB can be utilized for constructing magnetically driven microscale robots will undoubtedly stimulate interdisciplinary and cross‐domain technological innovation in various applications. The objective of this mini‐review is to provide an up‐to‐date overview of the recent advancements in MTB microrobots, delineate the interaction between MTB microrobots and magnetic fields, elucidate propulsion mechanisms and motion control, and report state‐of‐the‐art strategies for modifying and functionalizing MTB microrobots for biomedical applications (Figure 1 ). Following an overview of MTB as magnetic microrobots in the realms of biomedicine, including antitumor and antibacterial treatment, and detoxification, this review offers a forward‐looking perspective on this promising emerging field. At the same time, recognizing that the intrinsic magnetic components are a unique advantage of MTB compared to other microorganisms, we also briefly discuss the biomineralization mechanism as a fundamental component of this MTB‐focused review. A comprehensive understanding of MTB and magnetosome formation can facilitate improved design and fabrication of biomedical microrobots based on the characteristics of MTB.

Figure 1.

Schematic illustration of locomotion, targeting, functionalization, and biomedical applications of MTB microrobots. Adapted with permission.^{[ 43 ]} Copyright 2014, American Chemical Society. Adapted with permission.^{[ 44 ]} Copyright 2021, Wiley‐VCH GmbH. Adapted with permission.^{[ 45 ]} Copyright 2016, American Society for Microbiology. Adapted under the terms of the CC BY 4.0 license.^{[ 46 ]} Copyright 2021, MDPI. Adapted with permission.^{[ 47 ]} Copyright 2022, American Chemical Society. Adapted with permission.^{[ 48 ]} Copyright 2022, Elsevier.

2. Magnetotactic Bacteria and Biomineralization

2.1. Magnetotactic Bacteria

MTB, as ubiquitous aquatic Gram‐negative prokaryotes (Figure  2a), exhibit remarkable morphological, phenotypical, and physiological diversity. Their unique motility is orchestrated by the interactions between the Earth's magnetic field and their magnetosomes. They thrive in low‐oxygen environments, favoring sediments and chemically stratified water columns. Often congregating at the oxic‐anoxic interface (OAI), these bacteria have led some to speculate that they may represent Earth's early inhabitants. The pioneering observation of MTB was made by Salvatore Bellini in 1963 within the freshwater setting of Pavia, Italy.^{[ 51 ]} He noted a sizable bacterial population consistently swimming in a northward direction, postulating an internal compass as the cause. A decade later, Richard Blakemore independently elucidated the biological foundations of MTB's magnetotactic behavior.^{[ 52 ]} This phenomenon, known as magnetotaxis, involves cell migration along magnetic field lines, guided by the Earth's geomagnetic or external magnetic fields. Magnetotaxis is enabled by intracellular magnetosomes composed of nano‐sized magnetic iron mineral crystals. Arranged in chains parallel to the motility axis and encased by phospholipid bilayer membranes, these magnetosomes possess a magnetic dipole moment akin to a compass needle. Among freshwater MTB, a prevalent type of magnetic crystal is cuboctahedral magnetite (Fe₃O₄), with chains reaching lengths of up to 250 nm.^{[ 53 ]} Magnetite crystal shapes and sizes are generally consistent across strains, but smaller and rounder crystals often characterize the chain's terminus, indicating ongoing development. Another type of magnetic crystal, greigite (Fe₃S₄), exhibits more irregular morphologies and shorter chain lengths (35–120 nm).^{[ 54 ]} The bio‐mineralization process of greigite crystals is less constrained, resulting in scattered disordered clusters throughout the cytoplasm. This diminishes the uniformity of magnetic induction in magnitude and direction, yet maintains sufficient magnetic moment for movement along the field. Although studies predominantly focus on magnetite crystals, these magnetosomes have been associated with other functions such as iron homeostasis and the elimination of reactive oxygen species. Such insights underscore the multifaceted nature of MTB's capabilities.

Figure 2.

a) Typical structure of a magnetotactic bacterium. Adapted with permission.^{[ 49 ]} Copyright 2020, Springer Nature. b) Hypothesized mechanism of magnetite biomineralization to form magnetosomes. Adapted with permission.^{[ 50 ]} Copyright 2021, Springer Nature.

2.2. Biomineralization

The molecular understanding of magnetite crystal biomineralization in MTB is still quite limited, but progress has been made with preliminary experimental data. As shown in Figure 2b, iron uptake is hypothesized to be regulated and coupled to magnetite synthesis. Copper handling protein (ChpA), a copper‐containing periplasmic protein, forms an active oxidase‐permease complex to transport Fe (III) across the membrane by supplying copper to Fe(II) oxidase.^{[ 55 ]} Once iron is transported into the intracellular compartment, it reaches its destination in the magnetosome through specific routes. MamB and MamM are cation diffusion facilitator metal transporters that efflux divalent cations, including Fe (II), into the magnetosome.^{[ 56 ]} MagA protein has also been suggested to play a role in magnetosome transport, but experimental results remain ambiguous and lack verification with a variety of MTB species.^{[ 57 ]} With iron accounting for more than 4% of the intracellular dry weight, the harmful effects of the free intracellular iron and proton levels need to be mitigated and controlled. The proteins thought to be responsible for the elimination of toxic by‐products are MamN, which mediates active proton efflux, and MamT, which coordinates the iron redox reaction within the magnetosome. Following iron transportation into the magnetosome, MamG and MamD facilitate nucleation, and crystal growth occurs. The size of these crystals may be controlled by the two previously specified proteins in addition to MamF and MamC, all encoded on a single operon.^{[ 58 ]} Reduced oxygen levels are optimal for growth as smaller and irregularly shaped crystals tend to form at higher concentrations.^{[ 59 ]} After magnetite crystals are formed, the magnetosomes must align along the cell's curvature in a helical manner to maximize the total magnetic dipole that is the summation of each individual component.^{[ 60 ]} Developing crystals are loosely situated at the end of the chains and gradually concentrate more tightly at the mid‐line as they mature. To prevent the linear chain of magnetosomes from collapsing inward, resulting in a reduction of magnetostatic energy, the MamK protein forms the cytoskeletal magnetosome filament to support their structural arrangement.^{[ 61 ]} Magnetosomes are connected to the cytoskeletal filament through the MamJ protein interacts with MamK and itself.^{[ 62 ]} Deletion of these proteins results in different phenotypes with MamK, causing dispersion of magnetosomes and MamJ, creating compact clusters.^{[ 63} , ^{64 ]} Questions remain about what controls the dynamic localization and how magnetosomes are segregated during cell division.

3. Actuation Strategies of MTB Microrobots

It is well‐established that the magnetotactic behavior of MTB is closely linked to their aerotactic sensory mechanism, where movement along magnetic field lines aids their migration to a favorable oxygen state. This is likely explained by MTB's anaerobic properties (Figure 3a), as they tend to avoid high‐oxygen environments and prefer to be located at the OAI.^{[ 65 ]} In accordance with the aerotaxis response, the rotational direction of the helical flagella determines the migration direction.^{[ 66 ]} The polar magneto‐aerotaxis model, which uses a two‐state sensory mechanism, has been proposed to determine the flagella rotation and swimming direction. Under an oxidized state, a counterclockwise rotation occurs and propels the cell parallel to the magnetic field lines. When the oxygen concentration decreases under reducing conditions, the flagella switch direction, rotating clockwise, causing the migration direction to become anti‐parallel.^{[ 67 ]} By exploiting aerotactic and magnetotaxis behavior, we can actuate MTB, making directional control possible.

Figure 3.

Schematic illustration of the various actuation strategies of MTB microrobots. Adapted with permission.^{[ 40 ]} Copyright 2022, American Association for the Advancement of Science.

The MTB can also be actuated or guided by an external magnetic field. When a guiding field is applied, the MTB becomes magnetized and aligns itself with the field's direction. The motion of the MTB is then constrained in one direction, allowing for precise control (Figure 3b). When exposed to a strong magnetic gradient, the MTB will be magnetized and guided along the field, with the magnitude of the force being proportional to the gradient (Figure 3c). The magnetic field gradient method can be used to track and separate MTBs.^{[ 68} , ^{69 ]} However, the efficiency and maximum power achievable through this method are constrained by the strength of the magnetic gradient field. This could be more challenging when targeting deeper tissue, as the magnetic field gradient would decrease rapidly with increased distance from the source.^{[ 40} , ⁷⁰ , ^{71 ]} Alternatively, rotating or oscillating field‐driven methods have been developed to overcome the challenge of low Reynolds‐number hydrodynamics.^{[ 72 ]} When the MTB rotates in response to the external magnetic field, its inherent magnetic moment is unable to align with the external field fully due to the resistance caused by the surrounding fluid environment. This phase difference results in the generation of a magnetic torque on the bacteria, which is directly proportional to the angle of rotation (Figure 3d). When the frequency of the magnetic field is in the range of 10 to 25 Hz (at a strength of 20 mT, distance of 0.9 µm), the propulsion force generated by the rotating motion is ≈1 pN (achieving a comparable force on the same magnetic moment would demand 1300 T m⁻¹ gradient field). This propulsion force can potentially over‐drive the self‐propulsion of MTB when exposed to the external rotating magnetic field. Meanwhile, fixed MTBs can also be actuated using external magnetic fields. Although this approach sacrifices the self‐propelling advantage of living MTBs, it might provide enhanced controllability and still take advantage of their biological functions for medical operations.

4. Functionalization of MTB Microrobots

As live cells, modified bacteria can be used for theragnostic applications.^{[ 74 ]} Martel's research team pioneered the study of MTB surface functionalization and was the first to develop a simple and efficient method for linking MTB with carboxylated nanoliposomes. This linkage is achieved through carbodiimide cross‐linking chemistry, leveraging the abundance of naturally occurring amine groups on the MTB surface (Figure 4a).^{[ 43 ]} Similarly, Biotin‐PEG end‐modified with NHS can be efficiently immobilized onto the surface of MTB through amide bond formation,^{[ 46 ]} streamlining the carboxyl activation steps. Maleimides are also frequently employed for the site‐selective modification of proteins through coupling with thiol groups present on the protein. The Michael addition reaction between them exhibits remarkable selectivity and speed, under mild conditions.^{[ 75 ]} For example, Cai's research team initially used tris(2‐carboxyethyl) phosphine (TCEP) to reduce the surface protein disulfides of MTB (magnetospirillum magneticum AMB‐1) into reactive sulfhydryl groups. Then, a Michael addition reaction was achieved through the chemical conjugation of maleimide groups on the shells of light‐triggered indocyanine green nanoparticles (INPs) onto the activated sulfhydryl groups located on the outer membrane of AMB‐1 (Figure 4b).^{[ 44 ]} The combination of the hypoxia‐seeking AMB‐1, along with INP for both fluorescent imaging and photothermal therapy, suggests that this technique could be used as a targeted theranostic platform. In addition, research conducted by Song and colleagues encompasses a series of investigations pertaining to antibody‐coated MTB (Figure 4c).^{[ 45} , ⁷⁶ , ⁷⁷ , ^{78 ]} Their findings reveal that antibody coating facilitates the adhesion between MTB (magnetococcus massalia strain MO‐1) cells and Staphylococcus aureus (S. aureus). Magnetosome arrangements have also been investigated through genetic engineering methods; the involvement of MamJ in magnetosome chain assembly within the MTB (magnetospirillum gryphiswalense MSR‐1) has been established. Acting as a connector, MamJ binds magnetosomes to MamK filaments. In mutants with a deleted MamJ gene, magnetosomes detach from the MamK filament. Following this separation from the scaffold, the magnetosomes aggregate due to magnetic attraction (Figure 4d).^{[ 73 ]}

Figure 4.

Functionalization of MTB. a) Binding of nanoliposomes to the surface of MTB (MC‐1) by amide bond conjugation. Reproduced with permission.^{[ 43 ]} Copyright 2014, American Chemical Society. b) Coating MTB (AMB‐1) with the nanophotosensitizer INPs by maleimide‐thiol conjugation via a Michael addition reaction. Reproduced with permission.^{[ 44 ]} Copyright 2021, Wiley‐VCH GmbH. c) Attachment of antibody‐coated MTB (MO‐1) to S. aureus via affinity binding between S. aureus surface protein and MTB surface antibodies. Reproduced with permission.^{[ 45 ]} Copyright 2016, American Society for Microbiology. d) Magnetosome aggregation because of the separation of vesicles and filaments in mutant MTB. Reproduced with permission.^{[ 73 ]} Copyright 2006, Springer Nature.

5. Applications of MTB Microrobots

5.1. Bioimaging

MTBs have been extensively studied for their application in various imaging modalities due to their unique magnetic properties and biological adaptability. MTB serves as a natural contrast agent in magnetic resonance imaging, a technique that uses magnetic fields and radiofrequency pulses to visualize tissue structures with high resolution. Their intracellular magnetosomes, which exhibit superparamagnetic behavior, effectively enhance T₂‐weighted magnetic resonance imaging contrast, therefore enabling deep tissue imaging with high spatial resolution. As demonstrated in Figure 5a, Cai's research group first confirmed that the T₂ contrast signal intensity of AMB‐1 microrobots coupled with INPs (AI microrobots) exhibited an ideal linear relationship with the number of AMB‐1 bacteria.^{[ 44 ]} Subsequently, they observed significant signal intensities in T₂‐weighted MR images of tumors before and after injection, indicating effective accumulation of AMB‐1 triggered by AI microrobots and responsive degradation within the tumor microenvironment. The application of a magnetic field (AI+M) further enhanced the magnetic guidance of AI microrobots, directing them more to the tumor site. Similarly, magnetic particle imaging (MPI), a technique that directly detects superparamagnetic particles and provides quantitative images without signal attenuation at greater tissue depths, has benefited from MTB engineered with clustered magnetosome arrangements, such as the ΔmamJ mutant of MSR‐1. Contag's research group compared bioengineered MTB sourced from mutant and wild‐type specimens in the context of MPI (Figure 5b).^{[ 79 ]} They discerned that clusters of magnetosomes from MSR‐1 with the mamJ gene knocked out, as opposed to linear configurations from the wild‐type MSR1, elicit an augmented signal response. Delving into the distinctive rotations exhibited by these bioengineered nanoparticles owing to variances in shape or disposition holds promise for a deeper understanding of the underlying physics driving our imaging outcomes. Their findings elucidate the utility of MTB in the realm of MPI, thereby opening avenues for their utilization as living contrast agents in the domain of in vivo visualization and therapeutic intervention, as well as in diverse synthetic biological undertakings. These modified magnetosomes significantly improve MPI resolution and signal strength, making MTB ideal as living contrast agents.

Figure 5.

Bioimaging applications of MTB microrobots. a) Magnetic resonance imaging. b) Magnetic particle imaging. c) Fluorescence imaging. d) Bioluminescence imaging. e) Photothermal imaging. f) Magnetic hyperthermia imaging. Reproduced with permission.^{[ 79 ]} Copyright 2022, American Chemical Society. Reproduced with permission.^{[ 44 ]} Copyright 2021, Wiley‐VCH GmbH. Reproduced with permission.^{[ 47 ]} Copyright 2022, American Chemical Society.

For fluorescence imaging, which utilizes light‐emitting fluorophores for highly sensitive visualization of biological processes, also benefits from MTB. MTB modified with indocyanine green nanoparticles enable real‐time tracking and localization in superficial tissues, offering an effective means to monitor bacterial distribution in vivo (Figure 5c).^{[ 44 ]} Compared to the passive transport group with INPs, the active AI microrobots exhibited slight accumulation within tumor tissues due to their inherent anaerobic targeting properties. In contrast, tumors treated with AI+M showed a significant increase in total fluorescence signal within 24 h, with the fluorescence intensity being twice that of AI microrobots not exposed to a magnetic field. Bioluminescence imaging, an optical method based on light emitted from enzymatic reactions, utilizes MTB carrying the luxA‐E operon to provide non‐invasive and highly specific insights into bacterial viability and distribution (Figure 5d).^{[ 79 ]} Unveiling the temporal localization and integrity of this living contrast agent will advance its development as both an imaging tool and a bacterial therapeutic.

Photothermal imaging, based on the detection of heat generated by light‐absorbing agents under near‐infrared irradiation, is particularly advantageous for both imaging and therapeutic applications. MTB functionalized with photothermal agents like indocyanine green effectively highlights their accumulation sites while simultaneously enabling tumor ablation through localized heating (Figure 5e).^{[ 44 ]} Finally, magnetic hyperthermia, which employs alternating magnetic fields (AMF) to induce heat in magnetic materials, utilizes intact MTB for efficient heat generation and tumor targeting (Figure 5f).^{[ 47 ]} Compared to isolated magnetosomes, intact MTB demonstrate superior heating efficiency and antitumor activity, raising tumor temperatures to therapeutic levels while providing complementary imaging capabilities through MPI. Together, these six imaging modalities, categorized by their reliance on magnetic, optical, and thermal principles, highlight the versatility of MTB as natural, biocompatible, and multi‐functional imaging agents, promising significant advancements in diagnostic and therapeutic strategies.

5.2. Drug Delivery and Cancer Therapy

MTB microrobots, with their ability for autonomous motion and targeting of anaerobic regions, offer promising advancements in cancer treatment by actively delivering therapeutic payloads to tumor sites. To develop a new generation of therapeutic vectors based on MTB, Martel's research group attached nanoliposomes to the surface of MTB (Figure 6a). They investigated the linking efficacy, motility, and magnetic response of the MTB‐conjugated nanoliposomes (MTB‐LP) with each MTB effectively linking up to 70 nanoliposomes without losing its intrinsic motility and functionality.^{[ 43 ]} This study provides a model for the rational functionalization of MTB to meet the requirements of biomedical applications. To extend the application of nanoliposome‐attached MTB to the field of cancer treatment, the research team led by Martel pioneered and conducted relevant studies aiming to synthesize MTB into medical microrobots for precise tumor drug delivery. Their research group showed that when MC‐1 cells bearing covalently bound drug‐containing nanoliposomes were injected near the tumor and magnetically guided toward the tumor site, up to 55% of MC‐1 cells penetrated hypoxic regions of HCT116 colorectal xenografts (Figure 6b).^{[ 80 ]} Their findings indicate that leveraging swarms of MTB microrobots with magneto‐aerotactic migration behavior can substantially enhance therapeutic efficacy in hypoxic tumor regions.

Figure 6.

Anti‐tumor applications of MTB microrobots. a) Magnetic fields used to potentially deliver active substances to solid tumors through Nanoliposome‐attached MTB microrobots. Reproduced with permission.^{[ 43 ]} Copyright 2014, American Chemical Society. b) Delivery of drug‐containing nanoliposomes to tumor hypoxic regions by MTB microrobots. Reproduced with permission.^{[ 80 ]} Copyright 2016, Springer Nature. c) Tumor inhibition by activation of calcium influx via magneto‐mechanical stimulus exerted by MTB. Reproduced with permission.^{[ 48 ]} Copyright 2022, Elsevier. d) Magnetic hyperthermia tumor therapy of MTB microrobots under an alternating magnetic field. Reproduced with permission.^{[ 47 ]} Copyright 2022, American Chemical Society. e) Sequential magneto‐actuated and optics‐triggered MTB microrobots for targeted cancer therapy. Reproduced with permission.^{[ 44 ]} Copyright 2021, Wiley‐VCH GmbH. f) Magnetic torque–driven MTB living microrobots to increase tumor infiltration. Reproduced with permission.^{[ 40 ]} Copyright 2022, American Association for the Advancement of Science.

In addition to the directional motility via magnetotaxis and aerotaxis, the use of a dynamic magnetic field also shows promise for controllable propulsion and diverse anti‐tumor mechanisms such as ion influx activation and magnetic hyperthermia effect. For example, Song's research group devised an innovative approach utilizing MTB for cellular modulation and tumor suppression, employing a dynamic swing magnetic field (SMF) (Figure 6c).^{[ 48 ]} By introducing RGD peptides onto the surfaces of MTB cells, they successfully engineered these bacteria to recognize and adhere to mammalian tumor cells. The interaction between the elongated magnetosome chain within MTB bacterial cells and SMF generated magnetic torque. This torque induced pronounced swinging behaviors in the modified MTB bacteria adhered to the surfaces of tumor cells. Consequently, a sustained magnetomechanical oscillation was imparted onto the surfaces of tumor cells, leading to a substantial influx of Ca²⁺ in vitro and inhibition of tumor growth in vivo. These outcomes underscore the potential of magnetotactic bacteria‐mediated magnetomechanical stimulation, remotely orchestrated by dynamic magnetic fields, as a compelling approach for modulating cellular signaling and addressing tumor progression. Another therapeutic approach, magnetic hyperthermia of MTB, has been proven to hold enormous promise for cancer therapy from in vitro to in vivo by Song's research group (Figure 6d).^{[ 47 ]} When exposed to an alternating magnetic field, the complete AMB‐1 cell exhibits enhanced heating efficiency and achieves greater antitumor effectiveness compared to isolated magnetosomes or magnetosome chains.

In addition, the surface and interior of MTB can be further modified, allowing for artificial functionalization for use in antitumor therapeutic applications, as well as modification of magnetosomes for improved in vivo imaging. In one example, Cai fabricated a unique MTB microrobot for targeted cancer therapy,^{[ 44 ]} which was comprised of AMB‐1 MTB, enabling autonomous swimming toward tumors, and INPs, which serve as fluorescent imaging and photothermal agents (Figure 6e). The MTB microrobots could be tracked in vivo using fluorescence and magnetic resonance imaging, where they were found to migrate into tumors. Once they were localized to the tumor target, the solid tumors were effectively eradicated using photothermal therapy, when exposed to near‐infrared laser irradiation. This sequentially magneto‐actuated and optically triggered MTB microrobot offers a viable strategy for remote‐controlled targeted drug delivery, resulting in effective treatment of tumors.

The use of magnetic fields, in particular, can be used to precisely direct MTB microrobots. However, existing magnetic control strategies can be subject to limitations such as poorly scalable magnetic field gradients, active position feedback requirements, and are unsuitable for circulation throughout the body. Magnetic torque propulsion induced through a rotating magnetic field (RMF) exhibits great potential because this control strategy does not rely on the magnetic field gradient or active position feedback. These advantages enhance its scalability and make it more suitable for in vivo diffusion distribution, such as augmenting tumor penetration. Bhatia's research group used swarms of living MTB to act as micropropellers to induce the convection driven by a magnetic field.^{[ 81 ]} This magnetic responsive convection increases the transport efficiency of cargo into surrounding collagen matrices, indicating their potential to penetrate into tissue more effectively. Although a high density of MTB is required for effective convection, this challenge is surmountable if NPs are bound to the surface of MTB. Schuerle's research group has proposed a magnetic torque‐driven control scheme that can enhance the transport of MTB through biological barriers (Figure 6f).^{[ 40 ]} By culturing these biohybrid MTB microrobots, the researchers observed significantly increased bacterial accumulation in tumors following systemic intravenous injection in mice, providing promising results for their potential application within cancer treatment.^{[ 82} , ⁸³ , ^{84 ]}

5.3. Antimicrobial Treatment and Detoxification

Bacterial infections can give rise to severe consequences within healthcare facilities or societal settings. Conventional therapeutics rely on antibiotics, which could potentially increase drug resistance. In this context, MTB microrobots offer a promising alternative for mitigating bacterial infections. Through mechanisms such as mechanoporation or targeted drug delivery, MTB microrobots exhibit the potential to precisely and efficiently counteract bacterial growth (Figure 7a).^{[ 85 ]}

Figure 7.

Antimicrobial applications of MTB microrobots. a) Schematic diagram of MTB (MO‐1) functionalized with rabbit anti‐MO‐1 polyclonal antibody binding to S. aureus, inducing bacterial damage upon exposure to SMF. Reproduced with permission.^{[ 85 ]} Copyright 2017, Elsevier. b) SEM of MSR‐1 cells captured within a microtube. The inset shows a higher magnification view of the bacteria within the tube, with a scale bar of 500 nm. (Left). Bright‐field microscopy images of MSR‐1‐powered biohybrid swimming (Right). c) Magnetic guidance of biohybrids to E. coli biofilms. d) Increased magnification displays EPS and bacteria surrounding the biohybrid. Reproduced with permission.^{[ 86 ]} Copyright 2017, American Chemical Society. e) The migration of MO‐1 within a microfluidic chip under the influence of applied magnetic fields. f) The killing effect of antibody‐conjugated MO‐1 cells on S. aureus under the SMF. Reproduced with permission.^{[ 78 ]} Copyright 2017, Elsevier.

Biofilm colonies of bacteria typically exhibit resistance to standard antibiotic treatments, requiring tailored therapeutic strategies for treatment. Stanton et al. developed controllable micro‐swimmers (biohybrids) by combining non‐pathogenic MTB (MSR‐1) with drug‐loaded mesoporous silica microtubes.^{[ 86 ]} Figure 7b shows the partial penetration of MSR‐1 into the mesoporous silica microtubes (MSM). Once inside the MSM, the biorobots exhibit continuous directed motion. The surface‐bound (3‐aminopropyl)triethoxysilane (APTES) carries a positive charge, which may promote bacterial adhesion through van der Waals and electrostatic forces. The presence of APTES within the MSM induces cell adhesion, generates unidirectional forces from bacterial flagella, and allows the biorobots to swim effectively. These MTB microrobots were designed to deliver antibiotics specifically to target infectious biofilms. Unlike nanoparticles, which rely on random circulation in liquid to reach infection sites, the biorobots leverage the external magnetic guidance and motility capabilities of MSR‐1 cells to navigate and forcefully penetrate mature Escherichia coli (E. coli) biofilms (Figure 7c). Simultaneously, the biorobots utilize the acidic microenvironment within the biofilm to trigger the release of the antibiotic ciprofloxacin. This approach demonstrates the targeted and cargo release capabilities of non‐pathogenic bacterial species toward harmful biofilms, highlighting their tremendous potential in combating antibiotic‐resistant biofilms (Figure 7d).

Chen et al. assessed the bactericidal activity of MTB (MO‐1) against S. aureus in the presence of an SMF with low‐heat generation.^{[ 78 ]} This is a therapeutic tool that utilizes mechanical force, rather than temperature, to kill pathogens. Antibody‐coated MO‐1 cells were directed toward S. aureus using a focused magnetic field (Figure 7e), and once MO‐1 binds to S. aureus, a change in the SMF is induced. The SMF was tested in agar plated colonies of S. aureus (Figure 7f), with an application of 8 kPa of mechanical force to S. aureus resulting in cell death. With such a unique method, MTBs may pave the way for a future utilizing bacteria to treat bacteria.

With integrated biological functions, MTB microrobots are expected to function as neutralizers or have the ability to remove many potential toxins. This potential has recently been demonstrated in the removal of pollutants from water environments.^{[ 87} , ^{88 ]} Pollutants in solution are known to be lethal even at low concentrations. Current techniques are expensive and often fail to filter pollutants from water, particularly at low concentrations. MTBs address this challenge by offering an affordable and straightforward alternative. They naturally absorb metals and pollutants from the environment. By specifying and directing this behavior, MTBs could significantly reduce pollutant concentrations in solutions. Moreover, such approaches could be adapted for biodetoxification within the body for medical applications.

Further, MTB microrobots offer an efficient alternative to remove toxic organophosphate insecticides. J. Song et al. exploited the self‐propulsion and magnetically actuatable properties of MTB (AMB‐1) to bind to chlorpyrifos without additional supplementation.^{[ 87 ]} J. Song demonstrates the swarming technique where pesticide removal is made more efficient through directional manipulation (Figure 8a). There was 4 times higher chlorpyrifos removal when the MTB microrobots were swarming, versus when they were static, resulting in 86% removal of the pesticide from the solution after 180 min of treatment.

Figure 8.

Applications of MTB microrobots for toxin removal. a) Schematic diagram of MTB (AMB‐1) motion behaviors for pesticide removal. Maneuvering is achieved using a custom‐made controllable magnetic field. Reproduced under the terms of the CC BY 4.0 license.^{[ 87 ]} Copyright 2023, American Chemical Society. b) Scheme of magnetic separation of metal‐loaded MTB (KTN90) for the bioremediation processes. Reproduced with permission.^{[ 88 ]} Copyright 2016, Springer Nature.

MTB microrobot might also offer an efficient alternative to remove heavy metals. Other forms of bioremediation include the use of MTB (Alphaproteobacterium, KTN90), where Tajer‐Mohammed‐Ghazvini et al demonstrated its use to filter cobalt ions from solution (Figure 8b).^{[ 88 ]} Cobalt uptake is maximized by increasing the environmental temperature to 22–30 °C, thereby increasing the frequency of alterations between cobalt ions and MTB‐KTN90. Further optimization is obtained by fluctuation of the pH. At pH 7, optimal cobalt intake was observed due to fewer H⁺ ions being present to interfere with the intake of the positive cobalt ions. MTB‐KTN90 is especially efficient at cobalt absorption at low concentrations, and once enough cobalt is absorbed by MTB‐KTN90, it forms magnetosomes. Though the weight of cobalt absorbed is proportional to the initial concentration of cobalt in solution, the efficacy of cobalt sorption is greatest at the lowest concentration (1 mg L⁻¹). MTB microrobots offer a simple and effective method to remove such heavy metals.

6. Conclusion

In conclusion, the exploration of MTB microrobots and their applications, while still in its early stages, demonstrates remarkable potential across multiple domains through their unique capabilities in locomotion, sensing, imaging, and biological functions. The engineering of MTB as microrobots, particularly through precise magnetic manipulation, biological functionalization, and genetic modification, can pioneer the translational use of microrobotics in disease diagnostics and treatment. Understanding the biomineralization process that forms magnetosome chains within MTB could guide breakthroughs in genetic engineering and biotechnology, potentially extending magnetization capabilities to diverse cell types for microrobotic applications. In therapeutic applications, MTB microrobots offer transformative potential in several key areas. For drug delivery systems, their ability to move and navigate using magnetic guidance could enable highly targeted therapies, potentially reducing systemic side effects while enhancing drug efficacy at disease sites. Their potential as imaging agents could advance diagnostic capabilities while minimizing radiation exposure. MTB microrobots also hold great potential for minimally invasive surgery through remote magnetothermal ablation.

However, the translational use of MTB microrobots faces substantial technical and safety challenges. Critical technical barriers include optimizing MTB performance in complex in vivo environments, where varying oxygen levels, pH, and immune responses could affect their viability and functionality. Creating reliable control systems for the precise navigation of many MTB microrobots in complex biological environments presents another hurdle to overcome. Ensuring consistent stability and viability during storage and application remains challenging, as does developing scalable production methods that maintain bacterial properties, magnetosome quality, and therapeutic functions when loaded with medicine.

The safety and ethical considerations are equally complex and multifaceted. Primary concerns include potential pathogenicity and immune system activation, the risk of horizontal gene transfer to human microbiota, off‐target effects in non‐target tissues, environmental impact upon eventual disposal or release, and long‐term effects on human health and ecosystem balance. These challenges require a comprehensive approach to safety evaluation and risk mitigation, including rigorous pre‐clinical testing protocols, development of containment strategies, implementation of fail‐safe mechanisms, establishment of clear regulatory frameworks, and creation of standardized safety assessment guidelines. This includes ensuring informed consent through transparent communication of risks and benefits, maintaining equitable access to MTB‐based treatments, protecting patient rights during clinical trials, establishing clear protocols for environmental release and containment, and developing guidelines for responsible research and development.

Overall, the path forward requires a balanced approach that promotes technological innovation while maintaining rigorous safety standards and ethical considerations. Success in this field will depend on continued investment in fundamental research to understand MTB biology and behavior, coupled with the development of improved engineering techniques for MTB modification and control. Active engagement with experts in microbiology, synthetic biology, robotics, radiology, and clinics will be crucial. This holistic approach, combining scientific innovation with ethical responsibility, will be crucial for realizing the full potential of MTB microrobots while ensuring their safe and responsible implementation.

Conflict of Interest

The authors declare no conflict of interest.

Biographies

Jiaqi Wang received her Ph.D. degree in Chemistry from Nankai University and then joined the Department of Biomedical Engineering and Institute for Quantitative Health Science & Engineering at Michigan State University as a postdoctoral research fellow. Her major research interests focus on developing bioinspired materials, biosensors, and microrobotics.

Yi Xing is a postdoctoral research fellow in the Department of Chemistry at Indiana University Bloomington. Prior to this, he was a postdoctoral researcher at the Department of Biomedical Engineering and Institute for Quantitative Health Science & Engineering at Michigan State University from 2022 to 2023. He earned his Ph.D. in 2022 from the University of Science and Technology Beijing. His current research focuses on developing functional nanomaterials for biomedical applications, particularly in drug delivery and immune cell modulation.

Michael Ngatio is a Michigan State University graduate with a Bachelor of Science in Biochemistry and Molecular Biology. During his undergraduate studies, he was a research assistant for the Li Lab at the Institute for Quantitative Health Science and Engineering from 2022 to 2023, where he conducted research on hydrogels and microrobotics.

Paulina Bies is a graduate of Michigan State University, holding a Bachelor of Science in Computer Science. During her undergraduate studies, she worked as a research assistant in the Li Lab at the Institute for Quantitative Health Science and Engineering at Michigan State University from 2021 to 2024, where she conducted research in biomedical microrobotics.

Lu Lucy Xu is an MD/PhD student in biomedical engineering at Michigan State University College of Human Medicine. At Kurt Zinn's lab, her research focuses on alpha‐particle targeted radiation therapy for metastatic ovarian cancer. She received her MEng in Biomedical Engineering from the University of Toronto in 2021 and her Bachelor's degree in Medical Biophysics from Western University in 2020. Her prior work focuses on medical physics and radiation sciences.

Liuxi Xing is an MSU Innovations Venture Fellow and a research associate in the Department of Biomedical Engineering and the Institute for Quantitative Health Science & Engineering at Michigan State University. He received his Ph.D. degree in Biomedical Engineering from the City University of Hong Kong in 2022 and his M.S. degree in Mechanical Engineering from the University of Hong Kong in 2014. His research interests include micro‐mechanical systems, biomedical materials, and soft electronic systems.

Ahmed Zarea is a PhD student in the Cell and Molecular Biology Program at Michigan State University (USA). He earned his Bachelor's degree in Biology and Biochemistry from Mansoura University (Egypt) and worked as a research assistant at Michigan State University from 2018 to 2022. His research focuses on engineering genetically modified bacteria as engineered endosymbionts for therapeutic applications, particularly for delivering neural transcription factors to reprogram astrocytes into dopaminergic neurons for the treatment of Parkinson's disease.

Ashley Makela completed her PhD in Medical Biophysics (Molecular Imaging) at Western University in London, Canada, followed by a postdoctoral fellowship at the Institute for Quantitative Health Science and Engineering at Michigan State University. Her research involves using multimodal imaging to investigate the tumor microenvironment, and employing novel imaging techniques to validate the development of new therapeutics.

Christopher H. Contag is the James and Kathleen Cornelius Endowed Chair in the Departments of Biomedical Engineering and Microbiology Genetics & Immunology at Michigan State University. He is the Director of the Institute for Quantitative Health Science and Engineering at MSU. He is also a professor Emeritus in the departments of Pediatrics and Microbiology & Immunology at Stanford University, where he was on the faculty for 27 years. His research aims to control and image biology in complex living systems.

Jinxing Li is an Assistant Professor in the Department of Biomedical Engineering and the Institute for Quantitative Health Science and Engineering at Michigan State University. He earned his Ph.D. in NanoEngineering from the University of California San Diego, followed by a visiting scholarship at Nokia Bell Labs and postdoctoral training in the Department of Chemical Engineering at Stanford University. His research interests include microrobotics, bioelectronics, and living materials. He is a recipient of the NSF CAREER Award, DOE ARPA‐E IGNIITE Award, NIH Trailblazer Award, and Innovator Under 35 Global List by MIT Technology Review.

Wang J., Xing Y., Ngatio M., Bies P., Xu L. L., Xing L., Zarea A., Makela A. V., Contag C. H., Li J., Engineering Magnetotactic Bacteria as Medical Microrobots. Adv. Mater. 2025, 37, 2416966. 10.1002/adma.202416966